Gas-separation membranes are known and are typically employed for the separation of one or more components of a gaseous mixture. In general, gas-separation membranes may be employed usefully, for example, in the oxygen enrichment of air, for the removal of acid-gas components from natural, synthetic or refinery gases and for the separation and recovery of various gases, including hydrogen and carbon dioxide. Asymmetric, dry, cellulose acetate or cellulose ester-type membranes have been employed which have been suggested for use in the separation of helium from various gas mixtures, hydrogen from carbon monoxide and oxygen from nitrogen, through employing a dry cellulose-acetate membrane which comprises a thin, selected permeable layer and a relatively thick, integral porous layer (see, for example, U.S. Pat. No. 3,415,038, issued Dec. 10, 1968).
Cast separation membranes, typically useful for gas separation in spiral-module-type gas separators, have been prepared by casting a cellulose-acetate solution onto a shrinkable fabric and then gelling, leaching, annealing and drying by solvent exchange the cast film, to provide a fabric-supported, dry, cellulose ester-type membrane in which the shrinking of the fabric in the membrane is controlled, to avoid curling, wrinkling or cracking of the membrane in flat form or when spirally wound (see, for example, U.S. Pat. No. 4,134,742, issued Jan. 16, 1979). Asymmetric-type, cellulose-acetate membranes; that is, having a thin permeation layer and a thicker, relatively underlying porous layer, have been prepared and used typically for various ultrafiltration and reverse-osmosis processes, and such processes have been altered to provide for cellulose-acetate membranes particularly useful for gas-separation processes.
Generally, the preparation of a cellulose-acetate membrane comprises providing a casting solution of a solvent and cellulose acetate, casting the solution to form a membrane film, typically on a support, evaporating a portion of the solvent and then immersing the cast membrane in water, to form a wet, swollen, membrane-film structure and then drying and heat-annealing the swollen membrane, to reduce the porosity and to form a tight membrane structure suitable for reverse-osmosis processes (see U.S. Pat. No. 3,497,072, issued Feb. 24, 1970). One method for drying a water-wet cellulose-ester membrane comprises immersing the membrane in a water-soluble alcohol, such as isopropanol, to replace substantially all of the water in the membrane with the more volatile alcohol, and then immersing the alcohol-wet membrane in a nonpolar alcohol-soluble, volatile organic liquid, so as to replace the alcohol with the organic liquid, and then, thereafter, drying the organic liquid-wet membrane, to produce a dry cellulose-acetate membrane suitable for use in gas separation (see U.S. Pat. No. 3,842,515, issued Oct. 22, 1974). Another process for drying a water-wet membrane, particularly useful for gas membrane separation, comprises, firstly, contacting the water-wet membrane with a solution containing a major amount of one organic solvent and a minor portion of another organic solvent which is substantially miscible with the water, and, after the water has been removed substantially from the membrane, then evaporating the remaining replacement solution, to obtain a dry membrane free of water and the replacement solution (see, for example, U.S. Pat. Nos. 4,080,743 and 4,080,744, both issued Mar. 28, 1978, and U.S. Pat. No. 4,068,387, issued Jan. 17, 1978).
Dry, cellulose-ester, gas-separation membranes may be employed for a number of gas separations, such as, for example, for the removal of hydrogen sulfide and carbon dioxide from a natural gas stream, or to produce a carbon-dioxide-rich stream (see, for example, U.S. Pat. No. 4,130,403, issued Dec. 19, 1978). Thus, generally while cellulose-ester-type membranes may be employed for ultrafiltration and reverse-osmosis processes, in their use for gas separations, they typically must be dried by a special solvent-exchange process, to produce an effective gas-separation membrane.
Gas-separation membranes may be employed in various forms, such as in hollow-fiber, tubular, flat-sheet and spiral-module forms. Generally, spiral modules are mechanically simpler, easier to manufacture and are more effectively used in a membrane area, but also may be more prone to leaks and adhesive problems, but are generally preferred when the feed gas, and not the permeate gas, is the desirable product. Often, membranes employ the cellulose-ester or other polymer-type membranes on a support, such as a fabric support, and then are spiral-wound and are placed within a housing. Gas-separation membranes are also usefully employed in hollow fibers which are assembled into compact bundles, and which assembly provides for a large membrane area for gas transport. Such hollow-fiber bundles are typically assembled into a pressure vessel to form gas separators, and may be utilized into a skid-mounting system for easy field installation. The size of the housing for such hollow-fiber separators may vary by diameter and length and may be arranged in various configurations, depending on the gas to be separated and the number of separations required in the process. Such hollow-fiber gas separators comprise a pressure-type steel housing which contains a plurality of bundles and bundles of hollow fibers of the gas-separation membrane therein, with a fiber bundle plug at each end, and an inlet to introduce a feed stream of mixed gases to be separated, a gas outlet for the nonpermeate gaseous mixture and a permeate gas outlet to remove the gas which has permeated to the center within the hollow fibers of the bundles.
Hollow-fiber-type gas permeates are particularly useful for recovering hydrogen for ammonia purge gas, as well as for other gas-separation techniques (for example, see U.S. Pat. No. 4,172,885, issued Oct. 30, 1979, U.S. Pat. No. 4,180,552, issued Dec. 25, 1979, and U.S. Pat. No. 4,180,553, issued Dec. 25, 1979). The gas separators may be placed in a series of various permeate stages. Hollow fibers employed in the gas separators may be treated, such as by coating bundles of the hollow fibers (see U.S. Pat. No. 4,214,020, issued July 22, 1980).
A multiple-component-type membrane may be employed as a gas-separation membrane with a variety of gaseous mixtures (see, for example, U.S. Pat. No. 4,230,463, issued Oct. 28, 1980). This patent discloses a multicomponent gas-separation membrane comprising a porous, asymmetric substrate typically in hollow-fiber form made, for example, from a polymer, such as polysulfone, which has good, intrinsic, gas-separation properties, and which multicomponent membrane comprises a coating which has a high permeability for the gases, such as, for example, a coating of a silicone polymer. The polysulfone substrate is an effective separating barrier, as well as a physical support, while the silicone polymer material, which covers the polysulfone substrate, fills in the pores of the polysulfone membrane which constitutes imperfection in the polysulfone substrate. The added resistance to flow of the gas in the pores filled with the polysilicone polymer allows permeation in the much larger nonporous surface to predominate.
In the multicomponent membrane, it is important to select proper relative permeabilities for the coating and the substrate, since, if the permeability for the polysilicone polymer coating is much higher than the substrate, then too much of the gas mixture will go through the coating-filled pores. If the permeabilities of both the substrate and the coating are comparatively low, the resistance to flow of the gas will hamper productivity. Thus, the ratio of the coating permeability to that for the substrate is important, and the pores of the multicomponent membrane must be plugged with a material which exhibits a high permeability of the selected gas for the gaseous mixture to be separated by the membrane.
Such multicomponent membranes have been suggested for use to recover hydrogen from ammonia purge gas, to recover hydrogen from refinery desulfurization purge gas, to adjust the ratio of hydrogen to carbon monoxide from methanol and acetic-acid production and generally for the separation of fast-type gases from slow-type gases; that is, fast gases, such as hydrogen, helium, carbon dioxide, hydrogen sulfide and water, from slow gases, such as oxygen, methane, carbon monoxide, nitrogen and C.sub.2 -C.sub.6 aliphatic hydrocarbons.
In general, there are limitations both on the use of cellulose-ester-type dry, gas-separation membranes and multicomponent gas-separation membranes. Cellulose-ester-type membranes are variable in gas permeability and require a careful drying, to define the gas-separation properties. Cellulose acetate, either alone or with other cellulose-ester membranes, are subject to change under various process conditions, such as temperature, pH and other adverse conditions. The multicomponent gas-separation membranes, while useful, may be affected by impurities, and the membrane and the coating must be selected and adjusted for each specific gas separation. In addition, the allowable operating temperature range and the chemical resistance of the multicomponent membranes generally are limited by both the substrate and the coating material.
The conversion of cellulose-acetate-type membranes to gas-separation membranes, comprising separate casting, gelating, leaching and annealing and then the separate solvent-exchange drying, to provide the gas-separation membrane, often produces membranes of varying quality and with too great a variance in the gas-separation factor. Generally, the casting solution and the gelation conditions basically determine the intrinsic, high-flux, open-cell, asymmetric porous membrane, while the heat-annealing step tends to tighten the membrane skin, to produce the tight separation layer for reverse-osmosis membranes. Therefore, the problem arises of too great a variance with the separating factor, and a less asymmetric densification of the membrane tends to reduce permeation rate, while there are intrinsic problems for cellulose acetate regarding stability and aging affects in use. The multicomponent gas-separation membrane requires the selection of a particularly coated material and the balance of the intrinsic separation factors between the membrane and the coating material and the plugging of all of the membrane pores.
Thus, it is desirable to provide for an improved separation membrane and a method for making such a gas-separation membrane and a process of using such a membrane in the separation of fluids, particularly gases.